Copolyesterimides derived from aromatic dicarboxylic acids and aliphatic glycols and films made therefrom

ABSTRACT

A film comprising a copolyester wherein said copolyester comprises repeating units derived from an aliphatic glycol, an aromatic dicarboxylic acid, and a comonomer (M) selected from the group consisting of N,N′-bis-(2-hydroxyalkyl)-bicyclo-[2,2,2]-oct-7-ene-2,3,5,6-tetracarboxylic diimide (BODI) and 2-hydroxyalkyl-2-[p-(2-hydroxyethoxycarbonyl)phenyl]-1,3-dioxo-2H-isoindole-5-carboxylate (DOIC), wherein the number of carbon atoms is the 2-hydroxyalkyl group is 2, 3 or 4, and wherein comonomer (M) constitutes a proportion of the glycol fraction of the copolyester.

The present invention is concerned with novel polyesters and films madetherefrom, and methods for their synthesis. In particular, the presentinvention is concerned with novel copolymers of aromatic carboxylicacids, particularly copolymers of poly(alkylene naphthalate)s andcopolymers of poly(alkylene terephthalates), which exhibit improvedheat-resistance and thermo-mechanical stability.

The glass transition temperature (T_(g)), crystalline melting point(T_(m)) and degree of crystallinity are key parameters in determiningthe thermo-mechanical properties of polyesters. Previous studies havesucceeded in increasing the T_(g) of thermoplastic polymers, primarilyhomopolymers, but this has typically been accompanied by a correspondingincrease in the T_(m). Such increases in T_(m) can be disadvantageousbecause a thermoplastic polymer should also remain melt-processible (forinstance in an extruder), and should preferably remain so under economicconditions (for instance, below about 320° C., preferably below about300° C., which allows the use of conventional extrusion equipment). Athigher processing temperatures, polymer extrusion requires expensivespecialist equipment and a great deal of energy, and typically alsoresults in degradation products. The melt-processing temperature shouldbe well below (for instance, at least about 20° C. below) thedecomposition temperature of the polymer. In some cases, comonomers havebeen introduced into polymers in order to increase T_(g) while retainingT_(m), but also resulting in convergence of the decompositiontemperature and the T_(m), which leads to the production of degradationproducts in the melt.

Many attempts have also been made to enhance the glass transitiontemperature of polyesters by the introduction of more rigid comonomers.However, such comonomers also disrupt the packing of the polymer chainsin the crystal lattice, so that while the T_(g) increases, the T_(m) anddegree of crystallinity typically both decrease as the proportion ofcomonomer increases, leading ultimately to amorphous materials. In orderto fabricate articles from polymeric materials, it is often criticalthat the polymer exhibit crystallinity to achieve articles withacceptable thermo-mechanical properties.

Poly(ethylene terephthalate) (PET) is a semi-crystalline copolymerhaving a glass transition temperature (T_(g)) of 78° C. and acrystalline melting point of (T_(m)) of 260° C. Poly(ethylenenaphthalate) (PEN) is a semi-crystalline copolymer having a higher glasstransition temperature (T_(g)=120° C.) relative to PET, although theircrystalline melting points do not differ greatly (T_(m)=268° C. forPEN). The thermo-mechanical stability of PEN is significantly greaterthan that of PET. Many of the attempts made to enhance T_(g) by theintroduction of more rigid comonomers have focussed on PET, which issignificantly cheaper than PEN. There are no commercially availablesemi-crystalline polyesters with a T_(g) higher than PEN. Polyetherether ketone (PEEK) is one of the few examples of a high T_(g)(approximately 143-146° C.) semi-crystalline thermoplastic polymer, andhas been used successfully in engineering and biomedical applications.However, PEEK is suitable only for certain types of articles; forinstance, it is not suitable for the manufacture of biaxially orientedfilms. PEEK is also very expensive and has a high crystalline meltingpoint (approximately 350° C.).

An object of the present invention is to provide polyesters whichexhibit improved heat-resistance and thermo-mechanical stability. Afurther object of the present invention is to provide a thermoplasticpolymer with high or increased T_(g) but without increasing T_(m) to apoint where the polymer is no longer melt-processible under economicconditions (i.e. the polymer should remain melt-processible below about320° C., preferably below about 300° C.). A further object of thepresent invention is to provide semi-crystalline polyesters whichexhibit high T_(g) as well as high T_(m). A further object of thepresent invention is to increase the T_(g) of a polyester withoutsignificantly decreasing its T_(m) and/or its degree of crystallinity,and preferably without significantly decreasing its decompositiontemperature.

As used herein, the term “without significantly decreasing the T_(m)”means that the T_(m) decreases by no more than 10%, preferably no morethan 5%.

As used herein, the term “without significantly decreasing the degree ofcrystallinity”, means that the polyester retains a degree ofcrystallinity which is commercially useful, preferably in the range offrom about 10% to about 60%, preferably from about 20 to about 50%.

A further object of the present invention is to provide a copolyesterhaving a T_(g) which is higher than the corresponding base polyester,without significantly decreasing its T_(m) and/or its degree ofcrystallinity and preferably without significantly decreasing itsdecomposition temperature.

A further object of the present invention is to provide the use of acomonomer suitable for partial substitution of a monomer in aconventional polyester which increases the T_(g) of said polyesterwithout significantly decreasing its T_(m) and/or its degree ofcrystallinity, and preferably without significantly decreasing itsdecomposition temperature.

While the objects of the invention do not exclude an increase in T_(m),any increase in T_(m) must not be so large that melt-processing becomesuneconomical and that the T_(m) and decomposition temperature converge.

The above-mentioned objects of the present invention have, as theirunderlying objective, the provision of copolyester articles, andparticularly copolyester films, made from a copolyester having a T_(g)which is higher than the corresponding base polyester, withoutsignificantly increasing the T_(m) to a point where the polymer is nolonger melt-processible under economic conditions, particularly withoutsignificantly decreasing the degree of crystallinity of the article (inorder to achieve acceptable thermo-mechanical properties), andpreferably also without significantly decreasing decompositiontemperature.

As used herein, the term “copolyester” refers to a polymer whichcomprises ester linkages and which is derived from three or more typesof comonomers. As used herein, the term “corresponding base polyester”refers to a polymer which comprises ester linkages and which is derivedfrom two types of comonomers comprising ester-forming functionalities,and which serves as a comparator for a copolyester which is derived fromcomonomers comprising the comonomers of the corresponding basepolyester. A comonomer comprising ester-forming functionalitiespreferably possesses two ester-forming functionalities.

As used herein, the term “semi-crystalline” is intended to mean a degreeof crystallinity of at least about 5% measured according to the testdescribed herein, preferably at least about 10%, preferably at leastabout 15%, and preferably at least about 20%.

Accordingly, the present invention provides in a first aspect a filmcomprising a copolyester wherein said copolyester comprises repeatingunits derived from an aliphatic glycol, an aromatic dicarboxylic acid(preferably selected from terephthalic acid and naphthalene-dicarboxylicacid), and a comonomer (M) selected from the group consisting of themonomer of formula (I) and the monomer of formula (II):

wherein:

n=2, 3 or 4, and preferably wherein n=2; and

said comonomer (M) constitutes a proportion of the glycol fraction ofthe copolyester.

The monomer of formula (I) is referred to herein asN,N′-bis-(2-hydroxyalkyl)-bicyclo-[2,2,2]-oct-7-ene-2,3,5,6-tetracarboxylicdiimide (BODI). The copolyester comprising the monomer of formula (I)may also be referred to as a co(polyester-imide). Where n=2, the monomerof formula (I) has the formula (Ia):

The monomer of formula (II) is referred to herein as2-hydroxyalkyl-2-[p-(2-hydroxyethoxycarbonyl)phenyl]-1,3-dioxo-2H-isoindole-5-carboxylate(DOIC). Where n=2, the monomer of formula (II) has the formula (IIa):

Surprisingly, the present inventors have now found that incorporation ofthe specific co-monomer (M) into the polyester not only increases theT_(g) substantially but does so without significant detriment to thecrystallinity of articles made therefrom. This is achieved withoutsignificantly increasing the T_(m). The copolyesters according to thepresent invention are thermoplastic. Copolyesters and articles madetherefrom as described herein exhibit semi-crystalline properties. Thecopolyesters according to the present invention can be readily obtainedat high molecular weight. The copolyesters according to the presentinvention can be melt-processed below 320° C. (preferably below 300° C.)into tough, high strength articles.

The comonomer (M) constitutes a proportion of the glycol fraction of thecopolyester. In a preferred embodiment, the comonomer (M) is present inamounts of no more than about 50 mol % of the glycol fraction of thecopolyester, preferably no more than about 40 mol %, preferably no morethan about 30 mol %, preferably no more than about 20 mol %, preferablyno more than about 18 mol %, preferably no more than about 17 mol %, inone embodiment no more than about 15 mol % and in a further embodimentno more than about 10 mol %. Preferably the comonomer is present in anamount of at least about 1 mol %, more preferably at least about 3 mol%, more preferably at least about 4 mol % of the glycol fraction of thecopolyester.

The inventors have observed that even at low molar fractions of thecomonomer (M), small but valuable increases in T_(g) are observed. Forinstance, a copolyester comprising only 5 mol % comonomer (I) where n=2exhibits a significant rise in T_(g), while retaining a good degree ofcrystallinity.

The aromatic dicarboxylic acid is preferably selected from terephthalicacid and naphthalene-dicarboxylic acid. Other aromatic dicarboxylicacids which may be used in the present invention include isophthalicacid and phthalic acid. The naphthalene-dicarboxylic acid can beselected from 2,5-, 2,6- or 2,7-naphthalene dicarboxylic acid, and ispreferably 2,6-naphthalene dicarboxylic acid.

The aliphatic glycol is preferably selected from C₂, C₃ or C₄ aliphaticdiols, more preferably from ethylene glycol, 1,3-propanediol and1,4-butanediol, more preferably from ethylene glycol and 1,4-butanediol,and is most preferably ethylene glycol. The number of carbon atoms inthe aliphatic glycol may be the same or different as the number (n) inthe comonomer (M), but it is most preferably the same in order to retaincrystallinity, particularly in order to retain crystallinity withincreasing amounts of comonomer. Thus, the aliphatic glycol preferablyhas the formula HO(CH₂)_(m)OH, where m=n.

In one embodiment, the aliphatic glycol is 1,4-butanediol and n=4. In apreferred embodiment, the aliphatic glycol is ethylene glycol and n=2.

Copolyesters wherein the acid component is selected from 2,6-naphthalenedicarboxylic acid can be described by formula (III) below:

wherein:

n is as defined above;

p and q are the molar fractions of the aliphatic glycol-containingrepeating ester units and the monomer (M)-containing repeating esterunits, respectively, as defined hereinabove (i.e. q is preferably nomore than 50, and p=100−q);

the group X is the carbon chain of said aliphatic glycol; and

Z is selected from the group consisting of formula (Ib) and formula(IIb):

It will be appreciated that the single lines drawn from the N-atoms ateach end of the molecule in formula (Ib) and the single lines drawn fromthe O-atoms at each end of the molecule in formula (IIb) representsingle bonds to the (CH₂)_(n) groups in formula (III).

Copolyesters wherein the acid component is selected from terephthalicacid can be described by formula (IV) below:

wherein n, Z, X, p and q are as described above for formula (III).

The copolyester may contain more than one type of the aforementionedaliphatic glycols. Preferably, however, the copolyester comprises asingle type of the aforementioned aliphatic glycols. Where thecopolyester contains more than one type of said aliphatic glycols, thenpreferably the copolyester comprises a major aliphatic glycol fractionof a single type of said aliphatic glycols, and a minor aliphatic glycolfraction of one or more different type(s) of said aliphatic glycols,wherein said one or more different type(s) of said aliphatic glycolsconstitutes no more than 10 mol %, preferably no more than 5 mol %,preferably no more than 1 mol % of the total glycol fraction.

The copolyester comprising the comonomer (M) preferably comprises eitherthe monomer of formula (I) or the monomer of formula (II).

The copolyester may contain more than one type of monomer of formula (I)(i.e. a plurality of types of monomer (I) with differing values of n).Where the copolyester contains more than one type of said monomer offormula (I), then preferably the copolyester comprises a major fractionof a single type of said monomer of formula (I), and a minor fraction ofone or more different type(s) of said monomer of formula (I), whereinsaid minor fraction of one or more different type(s) of monomer offormula (I) constitutes no more than 10 mol %, preferably no more than 5mol %, preferably no more than 1 mol % of the total monomer (I)fraction. Preferably, the copolyester comprises a single type of monomerof formula (I).

Similarly, the copolyester may contain more than one type of monomer offormula (II) (i.e. a plurality of types of monomer (II) with differingvalues of n). Where the copolyester contains more than one type of saidmonomer of formula (II), then preferably the copolyester comprises amajor fraction of a single type of said monomer of formula (II), and aminor fraction of one or more different type(s) of said monomer offormula (II), wherein said minor fraction of one or more differenttype(s) of monomer of formula (II) constitutes no more than 10 mol %,preferably no more than 5 mol %, preferably no more than 1 mol % of thetotal monomer (II) fraction. Preferably, the copolyester comprises asingle type of monomer of formula (II).

Preferably, the copolyester comprises a single type of theaforementioned aliphatic glycols, and a single type of comonomer offormula (M). It will therefore be understood that preferred copolyestersof the present invention comprise a single type of the aforementionedaliphatic glycols, and either a single type of monomer of formula (I) ora single type of monomer of formula (II). The copolyesters may containminor amounts of other glycols and in a preferred embodiment such otherglycols constitute no more than 10 mol %, preferably no more than 5 mol%, preferably no more than 1 mol % of the total glycol fraction, but inorder to maximise performance it is preferred that the glycol fractionconsists of comonomer (M) and said aliphatic glycol(s) described above.

The copolyesters may contain more than one type of carboxylic acid. Inthis embodiment, the copolyester comprises a first aromatic dicarboxylicacid, which is preferably terephthalic acid or naphthalene-dicarboxylicacid, as described hereinabove, and one or more additional carboxylicacid(s). The additional carboxylic acid(s) is/are present in minoramounts (preferably no more than 10 mol %, preferably no more than 5 mol%, preferably no more than 1 mol % of the total acid fraction) andis/are different to said first aromatic carboxylic acid. The additionalcarboxylic acid(s) is/are preferably selected from dicarboxylic acids,preferably from aromatic dicarboxylic acids, for instance includingterephthalic acid (where the first aromatic dicarboxylic acid isnaphthalene-dicarboxylic acid), naphthalene-dicarboxylic acid (where thefirst aromatic dicarboxylic acid is terephthalic acid), isophthalicacid, 1,4-naphthalenedicarboxylic acid and 4,4′-diphenyldicarboxylicacid. In this embodiment, the first aromatic dicarboxylic acid may beone isomer of naphthalene-dicarboxylic acid, and the additionaldicarboxylic acid(s) may be selected from other isomer(s) ofnaphthalene-dicarboxylic acid. It will therefore be appreciated that theacid fraction of the copolyester preferably consists of a first aromaticdicarboxylic acid and optionally one or more additional aromaticdicarboxylic acid(s).

Preferably, however, the acid fraction consists of a single aromaticdicarboxylic acid as described hereinabove.

Thus, the copolyester preferably contains only aliphatic glycol(preferably a single aliphatic glycol; preferably ethylene glycol),aromatic dicarboxylic acid (preferably a single aromatic dicarboxylicacid; preferably terephthalic acid or naphthalene-dicarboxylic acid) andmonomer of formula (M) defined hereinabove.

The copolyesters can be synthesised according to conventional techniquesfor the manufacture of polyester materials by condensation or esterinterchange, typically at temperatures up to about 310° C.Polycondensation may include a solid phase polymerisation stage. Thesolid phase polymerisation may be carried out in a fluidised bed, e.g.fluidised with nitrogen, or in a vacuum fluidised bed, using a rotaryvacuum drier. Suitable solid phase polymerisation techniques aredisclosed in, for example, EP-A-0419400 the disclosure of which isincorporated herein by reference. In one embodiment, the copolyester isprepared using germanium-based catalysts which provide a polymericmaterial having a reduced level of contaminants such as catalystresidues, undesirable inorganic deposits and other by-products ofpolymer manufacture. In one embodiment, the aliphatic glycol is reactedwith the naphthalene dicarboxylic acid to form abis(hydroxyalkyl)-naphthalate, which is then reacted with the monomer(M) in the desired molar ratios under conditions of elevated temperatureand pressure in the presence of a catalyst, as exemplified in Scheme (1)hereinbelow. In a further embodiment, the aliphatic glycol is reactedwith the terephthalic acid to form a bis(hydroxyalkyl)-terephthalate,which is then reacted with the monomer (M) in the desired molar ratiosunder conditions of elevated temperature and pressure in the presence ofa catalyst, as exemplified in Scheme (2) hereinbelow.

The copolyesters described herein are particularly suitable for use inapplications involving exposure to high temperatures and applicationswhich demand high thermo-mechanical performance. One advantage of thecopolyesters described herein over PEEK is that they exhibit T_(g)values approaching those of PEEK, but with a T_(m) which issignificantly lower.

Surprisingly, the present inventors have found that incorporation of thespecific co-monomer (M) into an aromatic polyester (preferably aterephthalate or naphthalate polyester) not only increases the T_(g)substantially but does so without significant detriment to thecrystallinity of films made therefrom. This is achieved withoutsignificantly increasing the T_(m). Films made from the copolyestersdescribed herein exhibit semi-crystalline properties. Films made fromthe copolyesters described herein exhibit unexpectedly excellentsemi-crystalline properties. Semi-crystalline films of the inventionexhibit a degree of crystallinity of at least about 5%, preferably atleast about 10%, preferably at least about 15%, preferably at leastabout 20%, and preferably at least about 25%, measured according to thedensity method described herein. Thus, the present invention providesfilms wherein the aromatic dicarboxylic acid (or the first dicarboxylicacid as defined herein) is naphthalene dicarboxylic acid and the degreeof crystallinity of the film is at least about 5% (preferably 10%,preferably 15%, preferably 20%, preferably 25%) as calculated from thefilm density and on the basis of the density of 0% crystallinepolyethylene naphthalate (PEN) being 1.325 g/cm³ and the density of 100%crystalline PEN being 1.407 g/cm³; and further provides films whereinthe aromatic dicarboxylic acid (or the first dicarboxylic acid asdefined herein) is terephthalic acid and the degree of crystallinity ofthe film is at least about 5% (preferably 10%, preferably 15%,preferably 20%, preferably 25%) as calculated from the film density andon the basis of the density of 0% crystalline polyethylene terephthalate(PET) being 1.335 g/cm³ and the density of 100% crystalline PET being1.455 g/cm³.

The film of the present invention is preferably an oriented film,preferably a biaxially oriented film. Biaxially oriented films inparticular are useful as base films for magnetic recording media,particularly magnetic recording media required to exhibit reduced trackdeviation in order to permit narrow but stable track pitch and allowrecording of higher density or capacity of information, for instancemagnetic recording media suitable as server back-up/data storage, suchas the LTO (Linear Tape Open) format. The film (preferably biaxiallyoriented film) of the present invention is also particularly suitablefor use in electronic and opto-electronic devices (particularly whereinthe film is required to be flexible) where thermo-mechanically stablebackplanes are critical during fabrication of the finished product, forinstance in the manufacture of electroluminescent (EL) display devices(particularly organic light emitting display (OLED) devices),electrophoretic displays (e-paper), photovoltaic (PV) cells andsemiconductor devices (such as organic field effect transistors, thinfilm transistors and integrated circuits generally), particularlyflexible such devices.

The copolyester comprising repeating units derived from an aliphaticglycol, an aromatic dicarboxylic acid, and the monomer of formula (M) asdefined hereinabove is preferably the major component of the film, andmakes up at least 50%, preferably at least 65%, preferably at least 80%,preferably at least 90%, and preferably at least 95% by weight of thetotal weight of the film. Said copolyester is suitably the onlypolyester used in the film.

Formation of the film may be effected by conventional extrusiontechniques well-known in the art. In general terms the process comprisesthe steps of extruding a layer of molten polymer at a temperature withinan appropriate temperature range, for instance in a range of from about280 to about 300° C., quenching the extrudate and orienting the quenchedextrudate. Orientation may be effected by any process known in the artfor producing an oriented film, for example a tubular or flat filmprocess. Biaxial orientation is effected by drawing in two mutuallyperpendicular directions in the plane of the film to achieve asatisfactory combination of mechanical and physical properties. In atubular process, simultaneous biaxial orientation may be effected byextruding a thermoplastics polyester tube which is subsequentlyquenched, reheated and then expanded by internal gas pressure to inducetransverse orientation, and withdrawn at a rate which will inducelongitudinal orientation. In the preferred flat film process, thefilm-forming polyester is extruded through a slot die and rapidlyquenched upon a chilled casting drum to ensure that the polyester isquenched to the amorphous state. Orientation is then effected bystretching the quenched extrudate in at least one direction at atemperature above the glass transition temperature of the polyester.Sequential orientation may be effected by stretching a flat, quenchedextrudate firstly in one direction, usually the longitudinal direction,i.e. the forward direction through the film stretching machine, and thenin the transverse direction. Forward stretching of the extrudate isconveniently effected over a set of rotating rolls or between two pairsof nip rolls, transverse stretching then being effected in a stenterapparatus. Stretching is generally effected so that the dimension of theoriented film is from 2 to 5, more preferably 2.5 to 4.5 times itsoriginal dimension in the or each direction of stretching. Typically,stretching is effected at temperatures higher than the T_(g) of thepolyester, preferably about 15° C. higher than the T_(g). Greater drawratios (for example, up to about 8 times) may be used if orientation inonly one direction is required. It is not necessary to stretch equallyin the machine and transverse directions although this is preferred ifbalanced properties are desired.

A stretched film may be, and preferably is, dimensionally stabilised byheat-setting under dimensional support at a temperature above the glasstransition temperature of the polyester but below the meltingtemperature thereof, to induce the desired crystallisation of thepolyester. During the heat-setting, a small amount of dimensionalrelaxation may be performed in the transverse direction (TD) by aprocedure known as “toe-in”. Toe-in can involve dimensional shrinkage ofthe order 2 to 4% but an analogous dimensional relaxation in the processor machine direction (MD) is difficult to achieve since low linetensions are required and film control and winding becomes problematic.The actual heat-set temperature and time will vary depending on thecomposition of the film and its desired final thermal shrinkage butshould not be selected so as to substantially degrade the toughnessproperties of the film such as tear resistance. Within theseconstraints, a heat set temperature of about 150 to 245° C. (typicallyat least 180° C.) is generally desirable. After heat-setting the film istypically quenched rapidly in order induce the desired crystallinity ofthe polyester.

The film may be further stabilized through use of an in-line relaxationstage. Alternatively the relaxation treatment can be performed off-line.In this additional step, the film is heated at a temperature lower thanthat of the heat-setting stage, and with a much reduced MD and TDtension. The tension experienced by the film is a low tension andtypically less than 5 kg/m, preferably less than 3.5 kg/m, morepreferably in the range of from 1 to about 2.5 kg/m, and typically inthe range of 1.5 to 2 kg/m of film width. For a relaxation process whichcontrols the film speed, the reduction in film speed (and therefore thestrain relaxation) is typically in the range 0 to 2.5%, preferably 0.5to 2.0%. There is no increase in the transverse dimension of the filmduring the heat-stabilisation step. The temperature to be used for theheat stabilisation step can vary depending on the desired combination ofproperties from the final film, with a higher temperature giving better,i.e. lower, residual shrinkage properties. A temperature of 135 to 250°C. is generally desirable, preferably 150 to 230° C., more preferably170 to 200° C. The duration of heating will depend on the temperatureused but is typically in the range of 10 to 40 seconds, with a durationof 20 to 30 seconds being preferred. This heat stabilisation process canbe carried out by a variety of methods, including flat and verticalconfigurations and either “off-line” as a separate process step or“in-line” as a continuation of the film manufacturing process. Film thusprocessed will exhibit a smaller thermal shrinkage than that produced inthe absence of such post heat-setting relaxation.

The film may further comprise any other additive conventionally employedin the manufacture of polyester films. Thus, agents such asanti-oxidants, UV-absorbers, hydrolysis stabilisers, cross-linkingagents, dyes, fillers, pigments, voiding agents, lubricants, radicalscavengers, thermal stabilisers, flame retardants and inhibitors,anti-blocking agents, surface active agents, slip aids, gloss improvers,prodegradents, viscosity modifiers and dispersion stabilisers may beincorporated as appropriate. Such components may be introduced into thepolymer in a conventional manner. For example, by mixing with themonomeric reactants from which the film-forming polymer is derived, orthe components may be mixed with the polymer by tumble or dry blendingor by compounding in an extruder, followed by cooling and, usually,comminution into granules or chips. Masterbatching technology may alsobe employed. The film may, in particular, comprise a particulate fillerwhich can improve handling and windability during manufacture, and canbe used to modulate optical properties. The particulate filler may, forexample, be a particulate inorganic filler (e.g. metal or metalloidoxides, such as alumina, titania, talc and silica (especiallyprecipitated or diatomaceous silica and silica gels), calcined chinaclay and alkaline metal salts, such as the carbonates and sulphates ofcalcium and barium).

The thickness of the film can be in the range of from about 1 to about500 μm, typically no more than about 250 μm, and typically no more thanabout 150 μm. Particularly where the film of the present invention isfor use in magnetic recording media, the thickness of the multilayerfilm is suitably in the range of from about 1 to about 10 μm, morepreferably from about 2 to about 10 μm, more preferably from about 2 toabout 7 μm, more preferably from about 3 to about 7 μm, and in oneembodiment from about 4 to about 6 μm. Where the film is to be used as alayer in electronic and display devices as described herein, thethickness of the multilayer film is typically in the range of from about5 to about 350 μm, preferably no more than about 250 μm, and in oneembodiment no more than about 100 μm, and in a further embodiment nomore than about 50 μm, and typically at least 12 μm, more typically atleast about 20 μm.

According to a second aspect of the invention, there is provided anelectronic or opto-electronic device comprising the film (particularlythe biaxially oriented film) described herein, particularly electronicor opto-electronic devices such as electroluminescent (EL) displaydevices (particularly organic light emitting display (OLED) devices),electrophoretic displays (e-paper), photovoltaic (PV) cells andsemiconductor devices (such as organic field effect transistors, thinfilm transistors and integrated circuits generally), particularlyflexible such devices.

According to a third aspect of the invention, there is provided amagnetic recording medium comprising the film (particularly thebiaxially oriented film) described herein as a base film and furthercomprising a magnetic layer on one surface thereof. The magneticrecording medium includes, for example, linear track system data storagetapes such as QIC or DLT, and, SDLT or LTO of a further higher capacitytype. The dimensional change of the base film due to thetemperature/humidity change is small, and so a magnetic recording mediumsuitable to high density and high capacity causing less track deviationcan be provided even when the track pitch is narrowed in order to ensurethe high capacity of the tape.

According to a fourth aspect of the invention, there is provided acopolyester comprising repeating units derived from an aliphatic glycol,an aromatic dicarboxylic acid, and a comonomer (M) selected from thegroup consisting of the monomer of formula (I) and the monomer offormula (II) as defined hereinabove, wherein n=2, 3 or 4, whereincomonomer (M) constitutes a proportion of the glycol fraction of thecopolyester, and wherein said copolyester is semi-crystalline. Thegeneral and specific descriptions hereinabove of copolyesters appliesequally to copolyesters of the fourth aspect of the invention.Semi-crystalline copolyesters of the invention preferably comprise thecomonomer (M) present in amounts of at least about 3 mol %, preferablyat least about 4 mol %, preferably at least about 5 mol %, andpreferably no more than 20 mol %, preferably no more than 18 mol %,preferably no more than 17 mol %, preferably no more than 16 mol % andpreferably no more than 15 mol % of the glycol fraction of thepolyester. Semi-crystalline copolyesters of the invention exhibit adegree of crystallinity of at least about 5%, preferably at least about10%, preferably at least about 15%, and preferably at least about 20%,measured according to the standard DSC method described herein. Thedegree of crystallinity may be increased by annealing or SSP techniques.Annealing is conducted below the crystalline melting point (T_(m)) andabove the glass transition temperature (T_(g)) of the polymer, andpreferably at 20-80° C. below T_(m), and preferably from about 160 toabout 230° C. For copolyesters where the aromatic carboxylic acid (orthe first dicarboxylic acid) is terephthalic acid, preferred annealingtemperatures are in the range from about 160 to about 220° C. Forcopolyesters where the carboxylic acid (or the first dicarboxylic acid)is naphthalene-dicarboxylic acid, preferred annealing temperatures arein the range from about 180 to about 230° C. The annealing time ispreferably from about 30 minutes to about 4 hours, preferably from about1 to about 3 hours, and preferably about 2 hours. Annealing is conductedunder an inert atmosphere, preferably dry nitrogen.

According to a fifth aspect of the invention, there is provided acopolyester comprising repeating units derived from an aliphatic glycol,an aromatic dicarboxylic acid, and a comonomer (M) selected from thegroup consisting of the monomer of formula (I) and the monomer offormula (II) as defined hereinabove, wherein n=2, 3 or 4, whereincomonomer (M) constitutes a proportion of the glycol fraction of thecopolyester, wherein the acid fraction of the copolyester consists of afirst aromatic dicarboxylic acid and optionally one or more additionalaromatic dicarboxylic acid(s), and wherein the glycol fraction consistsof comonomer (M) and one or more aliphatic glycol(s). The copolyestersof the fifth aspect of the invention are otherwise as defined accordingto the description hereinabove of the copolyesters of the first aspectof the invention. The copolyesters of the fifth aspect of the inventionare preferably semi-crystalline, and the description hereinabove of thesemi-crystallinity of the copolyesters of the fourth aspect of theinvention applies equally to copolyesters of the fifth aspect of theinvention, including the preferred comonomer amount, degree ofcrystallinity and annealing process.

The copolyesters of the fifth aspect of the invention may be used tofabricate items in applications in which PEEK has been used, includingmechanical components (such as bearings, piston parts, pumps andcompressor plate valves); cable insulation; components for ultra-highvacuum applications; advanced biomaterials (including medical implants);and other applications in the aerospace, automotive, teletronic, andchemical process industries. The copolyesters of the fifth aspect of theinvention can be used in fibre form or in moulding compositions. Thecopolyesters of the fifth aspect of the invention, particularly thePET-based copolyesters, can also be used to manufacture bottles,particularly sterilisable and re-useable bottles. Thus, according to afifth aspect of the invention, there is provided a fibre or mouldingcomposition or moulded article comprising a copolyester of the fifthaspect of the invention. The fibre, moulding composition or mouldedarticle may be produced according to conventional techniques in the art.As used herein, the term “moulded articles” includes bottles.

The following test methods were used to characterise the properties ofthe novel compounds disclosed herein.

-   (i) Glass transition temperature (T_(g)); temperature of cold    crystallisation (T_(cc)), crystalline melting point (T_(m)) and    degree of crystallinity (X_(c)) were measured by differential    scanning calorimetry (DSC) using a TA Instruments DSC Q2000. Unless    otherwise stated, measurements were made according to the following    standard test method based on the method described in ASTM E1356-98.    The sample was maintained under an atmosphere of dry nitrogen for    the duration of the scan. A flow rate of 50 ml min⁻¹ and T_(zero) Al    pans were used. Samples of homopolymers and related copolymers (5    mg) were initially heated at 20° C. min⁻¹ from 20° C. to 350° C. in    order to erase the previous thermal history (1^(st) heating scan).    After an isothermal hold at 350° C. for 2 min, samples were cooled    at 20° C. min⁻¹ to 20° C. (1^(st) cooling scan). Samples were then    reheated at 20° C. min⁻¹ to 350° C. (2^(nd) heating scan). Values of    T_(g), T_(cc), and T_(m) were obtained from 2^(nd) heating scans,    whereas T_(c) was obtained from the 1^(st) cooling scans.

The value of T_(g) was determined as the extrapolated onset temperatureof the glass transition observed on the DSC scans (heat flow (W/g)against temperature (° C.)), as described in ASTM E1356-98.

The values of T_(c), T_(cc), and T_(m) were determined from the DSCscans as the peak exotherm or endotherm of their respective transitions.

Herein, the degree of crystallinity of the polymer is measured forsamples which have been annealed at 200° C. for 2 hours. The annealingof the sample was conducted during a DSC heating cycle using a TAInstruments DSC Q2000 under a nitrogen atmosphere. A flow rate of 50 mlmin⁻¹ and T_(zero) Al pans were used. Copolymer samples (5 mg) wereinitially heated at 20° C. min⁻¹ from 20° C. to 350° C. in order toerase the previous thermal history (1^(st) heating scan). After anisothermal hold at 350° C. for 2 min, samples were cooled at 20° C.min⁻¹ to 200° C. and held at this temperature for 2 h before beingcooled at 20° C. min⁻¹ to 20° C. (1^(st) cooling scan). Samples werethen reheated at 20° C. min⁻¹ to 350° C. (2^(nd) heating scan). Theexperimental enthalpy of fusion values (ΔH_(m)) were obtained from the2^(nd) heating scans.

The degree of crystallinity (X_(e)) was calculated according to theequation:

X _(c) =ΔH _(m) /ΔH _(m)°

wherein:ΔH_(m)=experimental enthalpy of fusion calculated from the integral ofthe melting endotherm;ΔH_(m)=theoretical enthalpy of fusion of the correspondingpoly(alkylene-carboxylate) homopolymer (i.e. without the comonomer offormula (M)) at 100% crystallinity. Thus, for copolyesters of thepresent invention comprising repeating units derived from ethyleneglycol, naphthalene-dicarboxylic acid and the co-monomer of formula (M),ΔH_(m)° is the theoretical enthalpy of fusion of a 100% crystalline PENpolymer (103 J/g), and for copolyesters of the present inventioncomprising repeating units derived from ethylene glycol, terephthalicacid and the co-monomer of formula (M), ΔH_(m)° is the theoreticalenthalpy of fusion of a 100% crystalline PET polymer (140 J/g), asdefined in the literature (B. Wunderlich, Macromolecular Physics,Academic Press, New York, (1976)).

-   (ii) Inherent viscosity (η_(inh)) was determined at 25° C. for 0.1%    w/v solutions of the polymer in CHCl₃/TFA (2:1) using a    Schott-Geräte CT-52 auto-viscometer, with capillary No. 53103.    Inherent viscosities were calculated as:

η_(inh) =ln[(t ₂ /t ₁)/c]

wherein:η_(inh)=Inherent Viscosity (dL/g)t₁=Flow time of solvent (s)t₂=Flow time of the polymer solution (s)c=Concentration of the polymer (g/dL)

-   (iii) Degree of crystallinity of the film is measured via    measurement of density. The density of the film samples is measured    using a calibrated calcium nitrate/water density column controlled    at a constant 23° C. using a water jacket using the following    method. Two 860 ml calcium nitrate solutions of known densities are    prepared, filtered and degassed in vacuo for 2 h before being pumped    simultaneously into a graduated column tube under hydrostatic    equilibrium. The two calcium nitrate solutions of known density are    low and high concentration solutions which form a range of densities    within the column to encompass the expected densities for the    semi-crystalline films of the present invention (corresponding to a    degree of crystallinity of from about 0 to about 60%, as defined by    the literature densities for the 0 and 100% homopolymers, as noted    below for the PET and PEN homopolymers). The concentration of each    solution is thus selected on the basis of the aromatic dicarboxylic    acid in the polymer (or where more than one dicarboxylic acid is    used, on the basis of the first aromatic dicarboxylic acid as    defined herein), and the solutions used are as follows.    -   PET: Low concentration solution: 1.28 g/cm³ (240.80 g calcium        nitrate; 860 mL water; 1.71 M molar concentration with respect        to calcium nitrate).    -    High concentration solution: 1.43 g/cm³ (369.80 g calcium        nitrate; 860 mL water; 2.62 M calcium nitrate).    -   PEN: Low concentration solution: 1.32 g/cm³ (275.20 g calcium        nitrate; 860 mL water; 1.95 M calcium nitrate).    -    High concentration solution: 1.41 g/cm³ (352.60 g calcium        nitrate, 860 mL water; 2.50 M calcium nitrate).

The density column is calibrated using eight pips of known density whichare washed in calcium nitrate solution before being placed in thegraduated column. For each pip placed in the column, the volume heightof the column is recorded upon reaching a constant level of suspension(after 4 to 5 hours). Separate measurements are taken for each pip togenerate a calibration plot of volume height against density. Themeasurement method is repeated for each film specimen (dimensions 3×5mm)and three specimens are used for each film sample to generate a mean ofthe measured volume height, from which the measured density(ρ_(recorded)) is obtained from the calibration plot. The degree ofcrystallinity (χ_(c)) is then calculated for each sample using Equation(1):

$\begin{matrix}{{\chi_{c}(\%)} = {100( \frac{\rho_{recorded} - \rho_{amorphous}}{\rho_{crystalline} - \rho_{amorphous}} )}} & (1)\end{matrix}$

where

-   χ_(c)=degree of crystallinity (%)-   ρ_(recorded)=recorded density of polymer (g cm⁻³)-   ρ_(amorphous)=known density of amorphous homopolymer (o%    crystallinity)-   ρ_(crystalline)=known density of 100% crystalline homopolymer.

The invention is further illustrated by the following examples. It willbe appreciated that the examples are for illustrative purposes only andare not intended to limit the invention as described above. Modificationof detail may be made without departing from the scope of the invention.

EXAMPLES

Reaction schemes to prepare copolyesters of the present invention wherecomomoner (M) is selected from monomer (I) are shown in Schemes 1 and 2below.

The preparation of copolyesters of the present invention where comomoner(M) is selected from monomer (II) follows similar pathways to thoseshown in Schemes 1 and 2 above. Thus, once the monomer (II) has beenprepared in accordance with the synthetic procedure described below toreach the compound corresponding to compound 1, the pathways are similarto those of Schemes 1 and 2 above.

Example 1 Synthesis of Monomer of Formula (I) (BODI)

Ethanolamine (11.76 g, 204 mmol) was added dropwise to a stirredsolution of bicyclo-[2,2,2]-oct-7-ene-2,3,5,6-tetracarboxylicdianhydride (24.00 g, 96.72 mmol) in DMF (˜250 mL). The solution washeated to 130° C. over a period of 1 h and left to reflux for 16 h. Thereflux was then stopped and the reaction mixture was cooled to 0° C.before being added to MeOH where an off-white precipitate formed. Theproduct BODI (29.25 g, 91%) was collected by filtration, dried in vacuoat 100° C. for 24 h and ground into a fine white powder. The compoundwas characterised using DSC, mass spectroscopy, NMR and IR spectroscopyas detailed below.

m.p. (DSC)=287° C. MS m/z=357.1050 [M+Na], calculated 357.0986. ¹H NMR(400 MHz, d⁶-DMSO) δ_(H) (ppm) 5.98 (2H, m, H_(a)), 4.81 (2H, br,H_(b)), 3.50 (8H, m, H_(c)), 3.33 (4H, m, H_(d)), 3.15 (2H, m, H_(e)).¹³C NMR (100 MHz, d⁶-DMSO) δ_(C) (ppm) 177.39 (C₁), 130.49 (C₂), 57.08(C₃), 42.12 (C₄), 38.88 (C₅), 33.23 (C₆). IR (ν_(max) cm ⁻¹) 3421 (O—Hstretch), 1681 (C═O stretch).

Example 2 Synthesis of Monomer of Formula (II) (DOIC)

A solution of 1,2,4-benzenetricarboxylic dianhydride (24.53 g, 0.128mol), 4-aminobenzoic acid (17.50 g, 0.128 mol) in DMF (˜250 mL) washeated to 130° C. over 2.5 h. The solution was left to cool to roomtemperature before being poured into distilled water where a yellowprecipitate formed. The intermediate DOIC product (16.60 g, 42%) wascollected by filtration, dried in vacuo at 80° C. for 24 h and groundinto a fine powder. A solution of intermediate DOIC product (16.45 g,52.85 mmol) and Mn(Ac)_(2.)4H₂O (0.10 g, 0.40 mmol) in ethylene glycol(˜250 mL) was heated to reflux for 4 h. The solution was left to cool toroom temperature before being poured into distilled water where a yellowprecipitate formed. The product DOIC (11.61 g, 55%) was collected byfiltration, dried in vacuo at 80° C. for 24 h and ground into a fineyellow powder. The compound was characterised using DSC, massspectroscopy, NMR and IR spectroscopy as detailed below.

m.p. (DSC)=266° C. MS m/z=400.1028 [M+H], calculated 400.0954. ¹H NMR(400 MHz, d⁶-DMSO) δ_(H) (ppm) 8.86 (1H, s, H_(a)), 8.64 (1H, s, H_(b)),8.23 (3H, m, H_(c)), 7.59 (2H, m, H_(d)), 4.70 (8H, m, H_(e)). ¹³C NMR(100 MHz, d⁶-DMSO) δ_(C) (ppm) 163.10 (C₁, C₂), 137.15 (C₃), 135.56(C₄), 134.80 (C₅), 131.25 (C₆), 129.54 (C₇), 126.84 (C₈), 126.56 (C₉,C₁₀), 125.89 (C₁₁), 124.78 (C₁₂), 67.38 (C₁₃), 62.16 (C₁₄). IR (ν_(max)cm⁻¹) 3367 (O—H stretch), 2953 (C—H stretch), 1707 (C═O stretch), 1217(C—O stretch).

Examples 3 to 13 Synthesis of the Copolyesters

Four sets of novel linear copolyesters were synthesised, bypolycondensation between either bis-(2-hydroxyethyl)-terephthalate(BHET) or bis-(2-hydroxyethyl)-2,6-naphthalate (BHEN) and the comonomersof either formula (I) or formula (II), in molar amounts of comonomerfrom about 5 to about 25 mol %. Copolymers containing varying amounts ofthe comonomer (M) (i.e. either comonomer (I) or comonomer (II)) wereobtained using Sb₂O₃ as catalyst. The general polyesterificationprocedure is as follows, wherein the amounts of reactants used areprovided in Tables 1 and 2 below. A stirred mixture of either BHET orBHEN and comonomer (M) and Sb₂O₃ (0.10 g, 0.34 mmol) was poured into aPC rig tube. The PC rig tube was lightly scored on the stem using aStanley blade to ensure safe extrusion and clamped inside a heatingblock. After being fitted with a polycondensation head, stirrer guide,air stirrer, delivery side arm, distillate tube inside an ice-filledDewar flask, thermocouples, optical revolution counter and connected toa gas manifold, the temperature was raised to 235° C. over 1 h under anitrogen purge. The air stirrer was then started with a pressure of 8.5psi, with the temperature maintained at 235° C. for 30 min. The nitrogenpurge was then stopped, with the system now under vacuum. The pressurewas gradually reduced to <5 mm Hg⁻¹ as the temperature was increased to280-290° C. at a rate of 1° C. min⁻¹. Once the viscosity of thesynthesised polymer had risen sufficiently to lower the stirrerrevolution rate by approximately 20-30 rpm, the copolymerisation wasadjudged to be complete. The vacuum was slowly replaced with a nitrogenpurge, allowing the synthesised copolymer to be extruded and quenchedinto an ice-water bath (1:1). The formed copolymer lace was left to dryin atmospheric conditions. The characterising data for the Examples aresummarised in Table 3 below.

TABLE 1 PET Copolymers Comonomer (M) Ex. Copolymer BHET (g) (BODI orDOIC) (g) 3 PETcoBODI-5 40.00 2.62 4 PETcoBODI-10 35.00 4.59 5PETcoBODI-15 35.00 6.88 6 PETcoBODI-20 30.00 7.86 7 PETcoDOIC-5 28.542.24 8 PETcoDOIC-10 35.00 5.48

TABLE 2 PEN Copolymers Comonomer (M) Ex. Copolymer BHEN (g) (BODI orDOIC) (g) 9 PENcoBODI-5 40.00 2.19 10 PENcoBODI-10 35.00 3.83 11PENcoBODI-15 35.00 5.75 12 PENcoBODI-20 35.00 7.67 13 PENcoDOIC-5 40.002.61

TABLE 3 Thermal and Viscosity Data T_(g) T_(cc) T_(m) ΔH_(m) XcViscosity Ex. Polymer (° C.) (° C.) (° C.) (J/g) (%) η_(inh) (dL g⁻¹) C1PET 75 160 257 44 31 — 3 PETcoBODI-5 90 — 244 51 36 0.50 4 PETcoBODI-10101 — 227 43 31 0.45 5 PETcoBODI-15 105 — — 16 11 0.43 6 PETcoBODI-20111 — — — — 0.40 7 PETcoDOIC-5 88 — 242 55 39 0.35 8 PETcoDOIC-10 90 —225 45 32 0.43 C2 PEN 119 191 267 36 35 — 9 PENcoBODI-5 125 — 253 49 480.43 10 PENcoBODI-10 132 — 239 40 39 0.49 11 PENcoBODI-15 139 — — 23 220.41 12 PENcoBODI-20 143 — — — — 0.45 13 PENcoDOIC-5 126 — 255 53 510.35

The control samples (C1) and (C2) are pure PET or PEN, synthesised inaccordance with the procedure described for Examples 3 to 13, butwithout the inclusion of the comonomer.

Samples of the copolymers can be oriented by hot-drawing to multipletimes their original dimensions. For example, fibres can be drawn afterheating the samples over a hotplate, thereby demonstrating thermoplasticbehaviour and drawing capability.

Biaxially oriented films can be manufactured from the copolymersdescribed above. The polymer is fed to an extruder (single screw; screwspeed approx. 80 rpm) at a temperature in the range of 275 to 300° C. Acast film is produced, which is electrostatically pinned and threadedaround the casting drum and over the top of the forward draw onto ascrap winder. Once settled, cast samples are collected at a range ofcasting drum speeds (2, 3 and 5 m\min) to give a range of thicknesses.The cast films are subsequently drawn using a Long Stretcher (suppliedby T.M. Long Co., Somerville, N.J.). The Long Stretcher comprises ahydraulically operated stretching head mounted inside a heated oven witha liftable lid. The operation of the stretching mechanism is based uponthe relative motion of two pairs of draw bars (one fixed and onemoveable, mounted normally to one another). The draw bars are attachedto hydraulic rams which control the amount (draw ratio) and speed (drawrate) of the imposed stretching. On each draw bar are mounted pneumaticsample clips attached to a pantograph system. A sample loading system isused to position samples within the pneumatic clips. A cast sample cutto a specific size (11.1×11.1 cm) is located symmetrically on a vacuumplate attached to the end of an arm. The arm is run into the oven andthe sample lowered so that it is between the clips. The clips are closedusing nitrogen pressure to hold the film and the loading arm withdrawn.The oven is heated to a specified temperature by two plate-heaters. Thelid is lowered and air heaters rapidly bring the sample up to aspecified temperature. After a suitable preheat time (typically 25-30seconds), the draw is manually initiated by the operator. A draw rate offrom about 2 cm/second to about 5 cm/second is typically used.Simultaneous biaxial draw in perpendicular directions is used in theseexamples. Suitable processing conditions are given in Table 4 below.

TABLE 4 Approx Air Heater Plate Heater Sample Draw Ratio Temp (° C.)Temp (° C.) PEN-based films 3.5 × 3.5 155 150 PET-based films 3.5 × 3.5100-120 100-120

The films produced on the Long Stretcher are then crystallised using aLaboratory Crystallisation Rig and held at specified temperatures(typically 150 to 240° C.) for specified times (typically from 2 to 100seconds). In this equipment, samples are clamped in a frame which isdropped pneumatically and held between heated platens for a specifictime before being rapidly quenched by dropping into iced water.

The density of the film samples is measured using a calibrated calciumnitrate/water density column controlled at a constant 23° C. using awater jacket.

Crystallinity of the PEN-based film samples is calculated using knownvalues for PEN density and crystallinity, on the basis of the followingliterature data:

-   Density of 0% crystallinity PEN=1.325 g/cm³-   Density of 100% crystallinity PEN=1.407 g/cm³

Crystallinity of the PET-based film samples is calculated using knownvalues for PET density and crystallinity, on the basis of the followingliterature data:

-   Density of 0% crystallinity PET=1.335 g/cm³-   Density of 100% crystallinity PET=1.455 g/cm³

1. A film comprising a copolyester wherein said copolyester comprisesrepeating units derived from an aliphatic glycol, an aromaticdicarboxylic acid, and a comonomer (M) selected from the groupconsisting of the monomer of formula (I) and the monomer of formula(II):

wherein n=2, 3 or 4, and wherein comonomer (M) constitutes a proportionof the glycol fraction of the copolyester, wherein the comonomer (M) ispresent in a range of from 1 to 18 mol % of the glycol fraction of thecopolyester, wherein the glycol fraction of the copolyester consists ofcomonomer (M) and one or more aliphatic glycol(s) selected from C₂, C₃and C₄ aliphatic diols, and wherein the acid fraction of the copolyesterconsists of a first aromatic dicarboxylic acid and optionally one ormore additional aromatic dicarboxylic acid(s).
 2. The film according toclaim 1 wherein comonomer (M) is selected from the monomer of formula(I).
 3. The film according to claim 1 wherein comonomer (M) is selectedfrom the monomer of formula (II).
 4. The film according to claim 1wherein the comonomer (M) is present in amounts of no more than 15 mol %of the glycol fraction of the copolyester.
 5. The film according toclaim 1 wherein the aliphatic glycol is selected from C₂, C₃ or C₄aliphatic diols.
 6. The film according to claim 1 wherein the aliphaticglycol is ethylene glycol.
 7. The film according to claim 1 wherein thenumber of carbon atoms in the aliphatic glycol is the same as the number(n) in comonomer (M).
 8. The film according to claim 1 wherein n=2. 9.The film according to claim 1 wherein the aromatic dicarboxylic acid isselected from naphthalene dicarboxylic acid and terephthalic acid. 10.The film according to claim 1 wherein the aromatic dicarboxylic acid is2,6-naphthalene dicarboxylic acid.
 11. The film according to claim 1wherein the copolyester has formula (III):

wherein: the group X is the carbon chain of said aliphatic glycol; p andq are the molar fractions of the aliphatic glycol-containing repeatingester units and the monomer (M)-containing repeating ester units,respectively; and the group Z is selected from the group consisting offormula (Ib) and formula (IIb):


12. The film according to claim 1 wherein the aromatic dicarboxylic acidis terephthalic acid.
 13. The film according to claim 1 wherein thearomatic dicarboxylic acid is terephthalic acid and the copolyester hasformula (IV):

wherein: the group X is the carbon chain of said aliphatic glycol; p andq are the molar fractions of the aliphatic glycol-containing repeatingester units and the monomer (M)-containing repeating ester units,respectively; and the group Z is selected from the group consisting offormula (Ib) and formula (IIb):


14. The film according to claim 1 which is an oriented film,particularly a biaxially oriented film.
 15. The film according to claim1 wherein said aromatic dicarboxylic acid is naphthalene dicarboxylicacid and the degree of crystallinity of the film is at least about 10%as calculated from the film density and on the basis of the density of0% crystalline polyethylene naphthalate (PEN) being 1.325 g/cm³ and thedensity of 100% crystalline PEN being 1.407 g/cm³; or wherein saidaromatic dicarboxylic acid is terephthalic acid and the degree ofcrystallinity of the film is at least about 10% as calculated from thefilm density and on the basis of the density of 0% crystallinepolyethylene terephthalate (PET) being 1.335 g/cm³ and the density of100% crystalline PET being 1.455 g/cm³.
 16. A copolyester comprisingrepeating units derived from an aliphatic glycol, an aromaticdicarboxylic acid, and a comonomer (M) selected from the groupconsisting of the monomer of formula (I) and the monomer of formula(II):

wherein n=2, 3 or 4; wherein comonomer (M) constitutes a proportion ofthe glycol fraction of the copolyester; wherein the comonomer (M) ispresent in a range of from 1 to 18 mol % of the glycol fraction of thecopolyester; wherein the acid fraction of the copolyester consists of afirst aromatic dicarboxylic acid and optionally one or more additionalaromatic dicarboxylic acid(s); and wherein the glycol fraction consistsof comonomer (M) and one or more aliphatic glycol(s) selected from C₂,C₃ and C₄ aliphatic diols.
 17. The copolyester according to claim 16wherein comonomer (M) is selected from the monomer of formula (I).
 18. Afibre or moulding composition or moulded article comprising acopolyester according to claim
 17. 19. The copolyester according toclaim 16 wherein comonomer (M) is selected from the monomer of formula(II).
 20. A fibre or moulding composition or moulded article comprisinga copolyester according to claim 19.